Probiotics are live microorganisms that, when administered in adequate amounts, confer a health benefit on the host. These products are now among the most popular dietary supplements with consumers. However, because traditional probiotic use has relied on dairy carrier matrices, the impact and equivalency of probiotics in dietary supplement formats are not well understood. This Administrative Supplement proposal is affiliated with my R01 project (1R01HD088428-01A1) investigating the efficacy and safety of Bifidobacterium animalis subsp. lactis (B. lactis) BB-12 (BB-12) in preventing pediatric antibiotic-associated diarrhea (AAD). Although prevention of AAD is one of the most common indications for probiotic treatment, the efficacy of many probiotic products used for AAD is not supported by rigorous independent research. In the R01 human study, patients will consume BB-12 in yogurt during antibiotic treatment and will be observed for the frequency and duration of diarrhea and adverse outcomes. A secondary hypothesis of the R01 is that administration of antibiotics will alter the gut microbiome and that BB-12 in yogurt will mitigate the disturbance that antibiotics might cause. However, because this is a human study, it is difficult to investigate the molecular mechanisms behind BB-12 effects in the gut and to involve costly comparisons of different carrier matrices. Therefore, this supplement will use a mouse model to address whether BB-12 consumed in yogurt is more efficacious than when given as a dietary supplement. By examining for changes to the fecal microbiota and intestinal environment following antibiotic and probiotic treatment, we will begin to elucidate the specific molecular mechanisms by which probiotics can reduce the risk for AAD. Primary Aim: To quantify colonic SCFA levels in mice receiving antibiotics alone or in combination with BB-12 in yogurt or supplement (powder) format. Secondary Aim: To assess the composition of the intestinal microbiota in mice using 16S rRNA profiling before, during, and after antibiotic consumption in mice receiving antibiotics alone or in combination with BB-12 in either yogurt or supplement (powder) format. Tertiary Aim: To assess the level of SCFA sensing in intestinal tissues in mice receiving antibiotics alone or in combination with BB-12 in either yogurt or supplement (powder) format. Primary Hypothesis: Administration of BB-12 in yogurt is more effective than capsule format at mitigating antibiotic-induced changes to the gut microbiome to prevent the development of AAD. Objective: To better understand probiotic carrier format in the prevention of AAD. Clinically, probiotics have been shown to significantly prevent AAD, but the mechanistic basis for the best mode of delivery to the digestive tract is unclear. Our long-term goal is to determine the impact of BB-12 on a variety of gastrointestinal disease states and ages, through high-level independent research. To improve our interpretation of the outcomes of our current Phase II study (R01), we need to first perform more mechanistic studies to understand how BB-12 can prevent AAD in either yogurt or dietary supplement format.